[Rust] Remove useless use of `format!`

[asfimport]

Running clippy on Arrow's Rust implementation shows a number of places where format! is being called when it isn't necessary.

Reporter: Bruce Mitchener / @waywardmonkeys
Assignee: Bruce Mitchener / @waywardmonkeys

PRs and other links:

• GitHub Pull Request #1849

_{Note: This issue was originally created as ARROW-2413. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 1849
#1849